1. Field of the Invention
The invention relates broadly to telecommunications. More particularly, the invention relates to methods and apparatus for redirecting common channel signalling system 7 (SS7) signalling messages through the asynchronous transfer mode (ATM) network.
2. State of the Art
Perhaps the most awaited, and now fastest growing technology in the field of telecommunications in the 1990's is known as Asynchronous Transfer Mode (ATM) technology. ATM is providing a mechanism for removing performance limitations of local area networks (LANs) and wide area networks (WANs) and providing bandwidth on the order of gigabits/second. Because ATM cells can carry many different kinds of data across a single backbone network, the ATM technology provides a unitary mechanism for broadband end-to-end telecommunications traffic.
Although the ATM network was originally conceived to eventually handle all types of data including ordinary narrowband voice telephone calls, its initial application was almost exclusively for the transport of broadband data transmissions. Initially, ATM technology was expensive to implement and existing narrowband TDM (time division multiplexing) technology was perfectly adequate for ordinary voice telephone calls. As ATM technology has been more and more accepted in recent years, there is now more incentive to migrate all communications traffic (including narrowband voice calls) onto the ATM network. The incentive to migrate all voice circuits onto the ATM network has been amplified by the popularity of the Internet. Existing narrowband telephone technology was designed to handle telephone calls which average four minutes in length. Today, with millions of people using narrowband voice circuits to connect via modem to the Internet, the average voice circuit call length has grown to well over twenty minutes. Thus, as "voice calls" become longer in length, the existing narrowband telephone technology becomes less adequate.
One challenge in migrating narrowband voice calls to the ATM network is that the signalling/addressing system used in the narrowband network for provisioning trunk lines is different from the signalling/addressing system used in the broadband ATM network for provisioning virtual circuits. The international standard signalling/addressing system for narrowband circuits is known as the Common Channel Signalling System No. 7 ("SS7", "Signalling System 7", or "C7").
Prior art FIG. 1 illustrates how narrowband SS7 signalling operates. The public switched telephone network (PSTN) 10 includes a plurality of switches, e.g. 12, 14, 16, 18 which are interconnected via trunk lines 20, 22, 24, 26, 28, 30. Generally speaking, switches are located at central offices (COs) and at other locations determined by the length and nature of the trunk lines connecting the switches. Some of the switches in COs are directly connected to subscribers (class 4/5 switches). Other switches (tandem switches) couple switches to other switches. As shown in FIG. 1 switch 12 is connected to a subscriber line 32 and switch 18 is connected to a subscriber line 34. When the subscriber at subscriber line 32 calls the subscriber at subscriber line 34, at least two switches (12 and 18) will be involved in the connection. Depending on the level of congestion in the network 10, the call may need to be routed from switch 12 to switch 14 and/or switch 16 before reaching switch 18. The switches which originate and terminate the call are referred to as service switching points (SSPs). Network traffic between SSPs may be routed via a packet switch called a Signal Transfer Point (STP).
In order for a call to be correctly setup, managed, and torn down, all of the switches on the network must be able to communicate with each other. According to the SS7 protocol, each switch is coupled to the SS7 network. In FIG. 1, the SS7 network is shown as 36. In particular, each switch is connected to the SS7 network by a single clear channel link, typically a DS0 or 64 k link. FIG. 1 shown links 38, 40, 42, and 44 coupling switches 12, 14, 16, 18, respectively to the SS7 network. According to the SS7 standard, the SS7 network may also include one or more centralized databases (not shown) referred to as Service Control Points (SCPs). In order for a call to be completed, it may be necessary for the originating SSP to consult an SCP in order to obtain routing information.
The hardware and software of the SS7 protocol are divided into functional abstractions called "levels" which map loosely to the Open Systems Interconnect (OSI) 7-layer model defined by the International Standards Organization (ISO). Prior art FIG. 2 illustrates the OSI reference model and the SS7 Protocol Stack.
The Message Transfer Part (MTP) is divided into three levels. The lowest level, MTP Level 1, is equivalent to the OSI Physical Layer. MTP Level 1 defines the physical, electrical, and functional characteristics of the digital signaling link. Physical interfaces defined include E-1 (2048 kb/s; thirty-two 64 kb/s channels), DS-1 (1544 kb/s; twenty-four 64 kb/s channels), V.35 (64 kb/s), DS-0 (64 kb/s), and DS-0A (56 kb/s). MTP Level 2 ensures accurate end-to-end transmission of a message across a signaling link. Level 2 implements flow control, message sequence validation, and error checking. When an error occurs on a signaling link, the message (or set of messages) is retransmitted. MTP Level 2 is equivalent to the OSI Data Link Layer. MTP Level 3 provides message routing between signaling points in the SS7 network. Each node in the SS7 network has a point code. Routing messages include the originating point code (OPC) as well as the destination point code (DPC). MTP Level 3 re-routes traffic away from failed links and signaling points and controls traffic when congestion occurs. MTP Level 3 is equivalent to the OSI Network Layer.
An SS7 message (carried in MTP Level 2) is called a signal unit (SU). There are three types of signal units: fill in signal units (FISU), link status signal units (LSSU), and message signal units (MSU). The FISUs are transmitted continuously unless other SUs are present. The LSSUs are used to control link alignment and to indicate the status of a signalling point, e.g. to signal an outage. The MSUs carry all call control, database query and response, network management, and network maintenance data.
The ISDN User Part (ISUP) defines the protocol used to set-up, manage, and release trunk circuits that carry voice and data between terminating line exchanges (e.g., between a calling party and a called party). ISUP is used for both ISDN and non-ISDN calls. However, calls that originate and terminate at the same switch do not use ISUP signaling. The basic messages used to setup and teardown a connection between switches include the CIC (circuit identification code). The CIC indicates the trunk circuit reserved by the originating switch to carry the call. The CIC is followed by one of the following message types: IAM (initial address message), ACM (address complete message), ANM (answer message), and REL (release message).
In some parts of the world (e.g., China, Brazil), the Telephone User Part (TUP) is used to support basic call setup and tear-down. TUP handles analog circuits only. In most countries, ISUP has replaced TUP for call management.
The Signaling Connection Control Part (SCCP) provides connectionless and connection-oriented network services and global title translation (GTT) capabilities above MTP Level 3. A global title is an address (e.g., a dialed 800 number, calling card number, or mobile subscriber identification number) which is translated by SCCP into a destination point code and subsystem number. A subsystem number uniquely identifies an application at the destination signaling point. SCCP is used as the transport layer for TCAP-based services.
The Transaction Capabilities Applications Part (TCAP) supports the exchange of non-circuit related data between applications across the SS7 network using the SCCP connectionless service. Queries and responses sent between SSPs and SCPs are carried in TCAP messages. For example, an SSP sends a TCAP query to determine the routing number associated with a dialed 800/888 number and to check the personal identification number (PIN) of a calling card user. In mobile networks (IS-41 and GSM), TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming.
All of the above layered protocol messages are sent across the SS7 network, not through the PSTN shown in FIG. 1. In contrast, a typical ATM network 50 is shown in prior art FIG. 3. The network 50 includes a plurality of ATM switches, e.g. 52, 54, 56, 58 which are linked together by broadband links (preferably SDH/SONET links), e.g. 60, 62, 64, 66, 68, 70. ATM switches are generally located at COs, at other locations such as satellite links, or even at customer premises. As mentioned above, until recently, the ATM network was used almost exclusively for broadband data communications. As such, FIG. 3 shows a data service unit (DSU) 72 coupled to the ATM switch 52 and a DSU 74 coupled to the ATM Switch 58.
Those skilled in the art will appreciate that the ATM PDU (protocol data unit) is called a cell and that it comprises fifty-three octets, with five octets being reserved for the cell header and the remaining forty-eight octets being used by the ATM Adaptation Layer (AAL) and the user payload. Once a connection is set up, the header of each ATM cell bound for that connection includes addressing information, namely a Virtual Path Identifier (VPI) and a Virtual Channel Identifier (VCI). The first four octets of a cell can be coded in a variety of formats to identify nonuser payload cells. One such convention is "metasignalling" which is used to establish a session with the network and negotiate session services via the ATM Network.
The ATM reference model, like the SS7 reference model, is based on the OSI reference model. Prior art FIG. 4 illustrates layers and planes of the ATM reference model on the left and examples of protocol placement on the right. While the physical layer may be almost any kind of network, the presently preferred medium for ATM traffic is the SDH/SONET optical network which provides the broadest bandwidth presently available. The ATM layer is responsible for managing the sending and receiving of cells, adding and processing the five octet cell header. The AAL is designed to support different types of applications and different types of traffic such as voice, video, and data. The higher layers are used to implement specific communications protocols for control, applications, and management. As seen best in the right hand side of FIG. 4, the AAL layer and higher layers are divided into three planes.
The control plane (C-plane) is used to setup connections in the ATM network using the Q.2931 signalling protocol in the higher layers. Below the Q.2931 signalling protocol lies the signalling ATM adaptation layer (SAAL). The SAAL supports the transport of Q.2931 messages between any two ATM switches which are running SVCs (switched virtual circuits). The SAAL contains three sublayers. The ATM adaptation layer common part (AAL CP) detects corrupted traffic transported across any interface using the C-plane procedures. The service specific connection-oriented part (SSCOP) supports the transfer of variable length traffic across the interface, and recovers from errored or lost service data units. The service specific coordination function (SSCF) provides the interface to the next upper layer, the Q.2931 signalling protocol.
The user plane (U-plane) contains user and application specific protocols such as TCP/IP, FTP, etc. The invocation of the U-plane protocols takes place only after the C-plane has set up a connection successfully or a connection has been pre-provisioned.
The management plane (M-plane) provides the required management services and is implemented with the ATM local management interface (LMI). The internet simple network management protocol (SNMP) and/or the OSI common management information protocol (CMIP) can also reside in the M-plane.
From the foregoing, it will be appreciated that the signalling protocols utilized in setup, tear down, and management of connections in the ATM network is very different from the protocols utilized by SS7 in the PSTN. As mentioned above, demand has recently grown for the carrying of voice over the ATM network. In response to that demand, various organizations such as the ITU-T and the ATM Forum have defined ATM Adaptation Level 2 (AAL2) standards and recommendations which are intended to integrate the carrying of voice data into the ATM scheme. While the AAL2 standard has been established, presently, there is very little commercial activity utilizing AAL2. This lack of activity is probably the result of the present requirements for the use of AAL2. In particular, presently, in order to utilize AAL2, the customer must provide the network with AAL2 type information in generating a call. Alternatively, a non-ATM type call may be carried in certain very limited circumstances by the ATM network by establishing for the user a PVC (permanent virtual channel) which carries all non-ATM voice data (i.e., there is a static map from the incoming narrowband call to an outgoing broadband call). However, these uses of AAL2 require either the purchase of specialized equipment by the user, or the maintenance of an expensive PVC link. True interworking for voice data between the narrowband network and the ATM network has not yet been established in the art.
3. Co-owned Technology
Previously incorporated co-owned applications Ser. Nos. 09/289,463 and 09/289,464 disclose a method and apparatus for mapping narrowband (DS0) voice circuits into AAL2 type SVCs and for generating AAL2 type ATM SETUP messages from SS7 and other SETUP messages. The DS0 data streams are coupled to an ATM switch via T1/E1 line interface modules and a voice server module (VSM). Details regarding the VSM are disclosed in co-owned Ser. Nos. 09/015,403 and 09/015,302, the complete disclosures of which are hereby incorporated by reference herein. The VSM is also coupled to the SS7 network so that call setup messages can be mapped to the ATM network. Collectively, the methods and apparatus of the previously incorporated co-owned applications are referred to as an interworking function (IWF) which is made part of an ATM edge switch and is typically co-located with and coupled to a class 4/5 switch in a CO. FIG. 5 illustrates a proposed arrangement for the IWF.
The components shown in FIG. 5 are taken from FIGS. 1 and 3 and bear the same reference numerals where appropriate. Thus a class 4/5 switch 12 servicing a subscriber line 32 and a class 4/5 switch 18 servicing a subscriber line 34 are the same as those shown in FIG. 1. These switches are coupled to the SS7 network 36 by signalling links 38 and 44 respectively. Similarly, the ATM switches 54 and 56 coupled to the ATM network 50 are the same as those shown in FIG. 3 except that each of these switches has been provided with an IWF (55 and 57 respectively). The IWF 55 is coupled via a trunk line 21 to the class 4/5 switch 12 and is coupled to the SS7 network via a DS0 signalling link 59. Similarly, the IWF 57 is coupled via a trunk line 29 to the class 4/5 switch 18 and is coupled to the SS7 network via a DS0A signalling link 61. Under this proposal, a call setup signal (IAM) from the switch 12 is sent via the link 38 to the SS7 network 36 which sends the signal via the link 59 to the IWF 55. The IWF 55 determines which ATM switch will be used to complete the call and sends Q.2931 connection control messages over the ATM network 50 to the ATM switch 56 to establish a SVC between the ATM switches to handle the call payload. The IWF 55 also determines which class 4/5 switch will be used to complete the call and sends a signal (IAM) vial the SS7 network to the switch 18. The switches communicate in this manner until a connection is completed. The payload for the call is mapped to and mapped from ATM cells by the IWF at both ends of the circuit. A disadvantage of this approach is that it requires many DS0 connections from ATM switches to the SS7 network and a point code for each. This can become very expensive. In addition to this disadvantage, there is also the possibility that the processing of SS7 signalling information within an ATM switch will be congested.
Another proposed way of implementing an IWF is to separate the functions of signalling mapping and payload mapping so that all signalling mapping is handled at a central location. FIG. 6 illustrates how this approach might be realized. According to this proposal, the IWF 55' and the IWF 57' only process payload. A central signalling IWF 63 is coupled to the ATM network via ATM switch 65 and is coupled to the SS7 network via a DS0 67. According to this approach, all SS7 signalling for calls which will be routed through the ATM network is processed by the IWF 63, converted into ATM signalling, and sent via switch 65 to the appropriate ATM switches in order to set up the SVC for the call. A disadvantage of this approach is that it is not scalable, i.e. the IWF 63 and switch 65 will soon be overloaded. Moreover, the processing of SS7 signalling information within the central signalling IWF and ATM switch will likely be congested frequently.